The invention relates to a compact detection device, for example to be used in gamma cameras. This device has applications in many fields, and particularly in nuclear medicine.
Gamma cameras are frequently used in the nuclear medicine field, and detection devices for these gamma cameras comprise a set of detectors adjacent to each other forming a detection plane supported by a substrate, each detector being connected to an electronic device for processing of the detected signal. For example, this type of device is described in patent application WO-96/20412. However, with this type of device, each gamma ray is identified solely by its amplitude and information is stored in the form of a voltage.
Other types of detection devices are known in the more general field of gamma cameras. In particular, one known technique is to use a detection device comprising several detectors placed side by side on a ceramic substrate, to form a detection plane. The ceramic substrate is designed to provide the mechanical strength of the overall device.
This type of detection device cannot operate unless it is polarized. This is achieved by providing the device with polarization means such as a resistance and a connecting capacitor; the resistance is grounded when the detector is connected to high voltage, and vice versa. These polarization means are placed on the surface of the ceramic substrate.
This detection device also comprises signal processing means for signals originating from detectors; these processing means are in the form of an ASIC (Application Specific Integrated Circuit) deposited on the substrate.
In this known detection device, the ceramic substrate is thin so that it is transparent to gamma rays that must be detected in the detection plane under the substrate. The substrate is thin such that the measurement ASIC can be located as close as possible to the detection plane, to minimize parasite capacitances on the link between the detection plane and the ASIC input.
Furthermore, it is known that polarization means for this type of device must be located on the shortest path between the detection plane and the ASIC. Furthermore, since the substrate is thin, the resistance and the polarization capacitor are implanted on the substrate surface by silk screen printing.
In particular, this type of detection device is described in the publication entitled: xe2x80x9cA Basic Component for ISGRI, the CdTe gamma camera onboard the INTEGRAL Satellitexe2x80x9d by M. ARQUES et al. The detection device described in this publication is applied to a satellite. In this particular case, the detection device comprises two detection stages for two energy ranges. A measurement ASIC is installed on this detection device to be close to the detectors in order to minimize parasite capacitances of the link between the detectors and the ASIC input. In this device, noise related to signal processing electronics used for the detected signals hinders detection of the signals. Furthermore, this noise may be high compared with the very low measured electrical charges. Furthermore, the noise and the signal to be measured are synchronous. Consequently, the measurement times and electronic processing times are sequenced; the signal processing electronics is inhibited during the photon measurement time. The measurement is memorized and then processed. If a photon appears on the detector during the processing time, it will be ignored. This operating method is particularly penalizing in nuclear medicine.
Many technical domains require the use of a gamma camera to efficiently detect signals with an amplitude similar to the amplitude of electronic noise. In this case, it is necessary to protect detectors from electronic disturbances caused by the detected signal processing electronics. In particular, this is the case in the field of nuclear medicine using semiconductor based detectors in which the signals to be detected are of the order of magnitude of a femto Coulomb.
In this case, the expert in the subject would typically use a ground plane to protect detectors from disturbances caused by electronic noise. However, adding a ground plane between the ceramic substrate and detectors creates a problem with the parasite capacitance added by this gamma plane that must have the smallest possible effect on the signal.
Furthermore, adding a ground plane between the ASIC and the detector on a conventional detection device as described previously, would increase the signal noise since the ceramic substrate on which the ASIC is fixed is thin.
Furthermore, an expert in the subject knows that the resistance/capacitor assembly must be placed on the shortest path between the detector and the ASIC. But it must not be located on the ground plane since the ground plane must be uniform. Furthermore, this resistance/capacitor assembly cannot be located in the ASIC due to parasite capacitances caused by the integration of components, and cannot be inserted between the detectors.
In the medical field, it is important that the detection device comprising the processing electronics should be as compact as possible with no dead areas (area not used for detection).
The purpose of the invention is to correct these disadvantages. Consequently, it proposes a compact detection device for a gamma camera to enable detection of signals with the order of magnitude of a femto Coulomb by introducing a ground plane inserted between two ceramic substrates.
More precisely, the invention relates to a compact detection device, for example for a gamma camera, comprising:
several detectors adjacent to each other to form a detection plane; and
a first ceramic substrate, characterized in that it comprises;
a second substrate on which the detectors are arranged;
means of polarizing the detection plane and the signal processing means for signals detected by the said detection plane, located in the first substrate; and
a ground plane placed between the first and the second substrates.
According to the invention, the second substrate may be made from a thick ceramic, the thickness of this ceramic being chosen so as to achieve a compromise between the quality of its mechanical strength and the parasite capacitance generated between the detection plane and the ground plane.
According to the invention, the first ceramic substrate may be thick so that it has a low permittivity and good mechanical properties.
Advantageously, the polarization means are integrated into the first substrate by a surface-mounted components assembly process.
Several compact detection devices may be placed adjacent to each other to make a large detection device without any dead areas.